Impish youngster having nearly all of shellfish (5)
I believe the answer is:
scamp
'impish youngster' is the definition.
(I've seen this before)
'nearly all of shellfish' is the wordplay.
'nearly all of' means to remove the last letter (I've seen 'nearly all' mean this).
'shellfish' becomes 'scampo' (scampo is a kind of shellfish).
'scampo' with its last letter taken off is 'SCAMP'.
'having' is the link.
(Other definitions for scamp that I've seen before include "Tearaway" , "villain" , "Mischievous child (colloq.)" , "Troublemaker" , "Scallywag" .)